Solution for Google Requirement to use GTIN for Variable Products
It's my understanding that Google Merchant will require GTIN for all products by 5/16/16. Stores selling Variable products with size/color variations will now need to create 20+ variations for each product and load the GTIN from there.

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Lee commented
For anyone else reading this, the Google Product Feed official extension supported this prior to the deadline, and still does today.
It provides a field for storing GTINs (or any of the supported feed attributes) which can be set either at the main product, or against individual variations.
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Dennis commented
This will solve the Problem, but WOO Support want more Votes till they will do. :(
http://ideas.woocommerce.com/forums/133476-woocommerce/suggestions/19876414-set-gtin-as-a-field-like-next-to-sku